Ferritic stainless steel and component for exhaust gas

ABSTRACT

This ferritic stainless steel has a predetermined chemical composition, at least one type of oxides of oxides containing 5 mass % or more of Al and oxides containing 5 mass % or more of Si are present on the surface, and, among the oxides present on the surface, the number of oxides having a diameter D of 0.1 μm or more and 2.0 μm or less is 10 or more per 93 μm 2 , the diameter D being represented by D=(Dmax+Dmin)/2 where Dmax is the maximum diameter of the oxide on the surface and Dmin is the minimum diameter of the oxide on the surface.

TECHNICAL FIELD

The present invention relates to ferritic stainless steel and a component for exhaust gas. More specifically, the present invention relates to ferritic stainless steel having excellent red scale resistance in a high-temperature water vapor atmosphere and a component for exhaust gas obtained using the ferritic stainless steel as a material.

Priority is claimed on Japanese Patent Application No. 2020-178302, filed in Japan on Oct. 23, 2020, the content of which is incorporated herein by reference.

BACKGROUND ART

Normally, stainless steel is heated up to a high temperature of 300° C. to 900° C. in the case of being used in applications such as exhaust gas path materials, stove combustion equipment, fuel cell members or plant-related materials. In addition, in the above-described applications, the stainless steel is used in environments where water vapor is contained, and thus there are cases where red scale (Fe-based oxide) is generated. This generated red scale does not only have a probability of scattering to adhere to and adversely affect other components depending on the case but also provokes a cause of thickness reduction by oxidation and a decrease in high-temperature strength.

Therefore, there is a desire for ferritic stainless steel having red scale resistance in high-temperature water vapor environments. Conventionally, a variety of methods for improving red scale resistance are known.

Patent Document 1 and Patent Document 2 describe that, when Si is added, diffusion of Cr is promoted, the amount of a Cr-based oxide formed is improved and an oxide coating is strengthened. Therefore, inventions described in Patent Document 1 and Patent Document 2 improve water vapor oxidation resistance and red scale resistance.

CITATION LIST Patent Document [Patent Document 1]

-   Japanese Unexamined Patent Application, First Publication No.     2003-160844

[Patent Document 2]

-   Japanese Unexamined Patent Application. First Publication No.     2003-160842

SUMMARY OF INVENTION Technical Problem

In the above-described related art, attention is paid to Cr and Si in steel, and the amounts of Cr and Si in steel are optimized. However, control of the addition of such alloying elements causes deterioration of manufacturability and an increase in cost. Therefore, the present inventors studied improvement in red scale resistance by methods other than the addition of the alloying elements. Specifically, the present inventors paid attention to oxides having a specific composition on the surface of stainless steel in order to improve red scale resistance.

An objective of the present invention is to provide ferritic stainless steel having excellent red scale resistance and a component for exhaust gas having excellent red scale resistance for which the ferritic stainless steel is used as a material.

Solution to Problem

The gist of the present invention for solving the above-described problem is as described below.

[1] A ferritic stainless steel according to one aspect of the present invention contains, as a chemical composition, 0.05 mass % or more and 2.50 mass % or less of Si, 0.05 mass % or more and 1.50 mass % or less of Mn, 0.025 mass % or less of C, 0.040 mass % or less of P, 0.003 mass % or less of S, 0.025 mass % or less of N, 0.01 mass % or more and 0.50 mass % or less of Ni, 10.50 mass % or more and 25.00 mass % or less of Cr, 0.01 mass % or more and 1.80 mass % or less of Cu, 0.002 mass % or more and 0.200 mass % or less of Al, 0.001 mass % or more and 1.00 mass % or less of Nb, 0 mass % or more and 2.5 mass % or less of W, 0 mass % or more and 3.00 mass % or less of Mo, 0 mass % or more and 0.500 mass % or less of Ti, 0 mass % or more and 0.0100 mass % or less of B, 0 mass % or more and 0.0030 mass % or less of Ca, 0 mass % or more and 0.50 mass % or less of Hf, 0 mass % or more and 0.40 mass % or less of Zr, 0 mass % or more and 0.50 mass % or less of Sb, 0 mass % or more and 0.30 mass % or less of Co, 0 mass % or more and 1.0 mass % or less of Ta, 0 mass % or more and 1.00 mass % or less of Sn, 0 mass % or more and 0.30 mass % or less of Ga, 0 mass % or more and 0.50 mass % or less of V, 0 mass % or more and 0.003 mass % or less of Mg and 0 mass % or more and 0.20 mass % or less of REM and a remainder including Fe and an impurity, at least one type of oxides of oxides containing 5 mass % or more of Al and oxides containing 5 mass % or more of Si are present on a surface, and, among the oxides present on the surface, the number of oxides having a diameter D of 0.1 μm or more and 2.0 nm or less is 10 or more per 93 μm², the diameter D being represented by the following formula (1)

D=(Dmax+Dmin)/2  (1)

(In the formula (1), Dmax is a maximum diameter of the oxide on the surface, and Dmin is a minimum diameter of the oxide on the surface.)

According to the above-described configuration, it is possible to realize ferritic stainless steel having excellent red scale resistance.

[2] The ferritic stainless steel according to [1], in which, regarding the surface, CIE1976 luminosity L* that is measured using a D65 light source in a diffuse lighting mode by receiving light in a direction at 8° with respect to a normal line to the surface at a visual field angle of 10° visual field for a measurement time of one second may be 60 or more.

According to the above-described configuration, it is possible to realize ferritic stainless steel having excellent designability.

[3] The ferritic stainless steel according to [1] or [2], in which the chemical composition may contain one or more of 0.01 mass % or more and 2.5 mass % or less of W, 0.01 mass % or more and 3.00 mass % or less of Mo, 0.001 mass % or more and 0.500 mass % or less of Ti, 0.0002 mass % or more and 0.0100 mass % or less of B, 0.0002 mass % or more and 0.0030 mass % or less of Ca, 0.001 mass % or more and 0.50 mass % or less of Hf, 0.01 mass % or more and 0.40 mass % or less of Zr, 0.005 mass % or more and 0.50 mass % or less of Sb, 0.01 mass % or more and 0.30 mass % or less of Co, 0.001 mass % or more and 1.0 mass % or less of Ta, 0.002 mass % or more and 1.00 mass % or less of Sn, 0.0002 mass % or more and 0.30 mass % or less of Ga, 0.01 mass % or more and 0.50 mass % or less of V, 0.0003 mass % or more and 0.003 mass % or less of Mg and 0.001 mass % or more and 0.20 mass % or less of REM.

According to the above-described configuration, it is possible to improve the workability, high-temperature strength, corrosion resistance and oxidation resistance of steel sheets and the secondary workability or the like of formed articles that are manufactured using the ferritic stainless steel.

[4] The ferritic stainless steel according to any one of [1] to [3], in which, in a range from the surface to 1.0 μm after the ferritic stainless steel is held in an atmosphere of 300° C. to 900° C. for 100 hours or longer, when a Cr content is represented by [Cr], a Si content is represented by [Si] and an Al content is represented by [Al] by unit mass %, the following formula (2) may be satisfied.

[Cr]+[Si]+[Al]≥18.0  (2)

[5] In ferritic stainless steel according to another aspect of the present invention, in a range from a surface to 1.0 μm, when a Cr content is represented by [Cr], a Si content is represented by [Si] and an Al content is represented by [Al] by unit mass %, the following formula (2) is satisfied.

[Cr]+[Si]+[Al]≥18.0  (2)

[6] A component for exhaust gas according to one aspect of the present invention contains the ferritic stainless steel according to any one of [1] to [5].

Advantageous Effects of Invention

According to the above-described aspects of the present invention, it is possible to provide ferritic stainless steel having excellent red scale resistance and a component for exhaust gas having excellent red scale resistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart showing an example of a manufacturing method of ferritic stainless steel according to one embodiment of the present invention.

FIG. 2A is a view showing an example of a SEM photograph of a surface of ferritic stainless steel of a steel material No. 6 of an example.

FIG. 2B is a view showing an example of a SEM photograph of a surface of ferritic stainless steel of a steel material No. 8 of an example.

DESCRIPTION OF EMBODIMENTS

Hereinafter, ferritic stainless steel according to one embodiment of the present invention (ferritic stainless steel according to the present embodiment), a manufacturing method of ferritic stainless steel according to the present embodiment and a component for exhaust gas that is obtained using the ferritic stainless steel according to the present embodiment as a material (component for exhaust gas according to the present embodiment) will be described. The following description is intended to make the gist of the invention better understandable and does not limit the present invention unless particularly otherwise described. In addition, in the present specification, “A to B” indicates A or more and B or less.

In addition, in the present specification, the term “stainless steer” means a stainless-steel material for which a specific shape is not limited. Examples of this stainless-steel material include a steel sheet, a steel pipe, a steel bar and the like.

<<Ferritic Stainless Steel>> <Chemical Composition>

The composition of components (chemical composition) that ferritic stainless steel contains in one embodiment of the present invention is as described below. The ferritic stainless steel is formed of, in addition to each component to be described below, iron (Fe) and/or a small amount of an impurity (impurity) that can be incorporated from a raw material or in a manufacturing step.

(Chromium: Cr)

Cr is an essential element for forming a passive film and ensuring corrosion resistance. In addition, Cr is also effective for ensuring red scale resistance. In order to obtain these effects, the Cr content is 10.50 mass % or more. The Cr content is preferably 12.50 mass % or more.

On the other hand, when Cr is excessively contained, the material cost increases, and Cr becomes a cause of toughness deterioration. Therefore, the Cr content is 25.00 mass % or less. The Cr content is preferably 23.00 mass % or less.

(Silicon: Si)

Si is an effective element for improving red scale resistance. In order to obtain this effect, the Si content is 0.05 mass % or more. The Si content is preferably 0.10 mass % or more.

On the other hand, when Si is excessively contained. Si becomes a cause of deterioration of toughness or workability. Therefore, the Si content is 2.50 mass % or less. The Si content is preferably 2.00 mass % or less.

(Copper: Cu)

Cu is an element that is contained to ensure high-temperature strength. In order to obtain this effect, the Cu content is 0.01 mass % or more. The Cu content is preferably 0.02 mass % or more.

On the other hand, when Cu is excessively contained, a ferrite phase becomes unstable, and the material cost increases. Therefore, the Cu content is 1.80 mass % or less. The Cu content is preferably 1.60 mass % or less.

(Niobium: Nb)

Nb is an element that is contained to ensure high-temperature strength. In order to obtain this effect, the Nb content is 0.001 mass % or more. The Nb content is preferably 0.05 mass % or more and more preferably 0.10 mass % or more.

On the other hand, when Nb is excessively contained, there is a probability that workability and toughness deteriorate. Therefore, the Nb content is 1.00 mass % or less. The Nb content is preferably 0.70 mass % or less and more preferably 0.45 mass % or less.

(Manganese: Mn)

Mn is an element that improves the adhesion of scale in ferritic stainless steel. In order to obtain this effect, the Mn content is 0.05 mass % or more. The Mn content is preferably 0.10 mass % or more.

On the other hand, when Mn is excessively contained, a ferrite phase becomes unstable, and the formation of MnS, which acts as a corrosion starting point, is promoted. Therefore, the Mn content is set to 1.50 mass % or less. The Mn content is preferably 1.20 mass % or less.

(Nickel: Ni)

Ni is an element that improves the corrosion resistance of ferritic stainless steel. In order to obtain this effect, the Ni content is 0.01 mass % or more. The Ni content is preferably 0.05 mass % or more.

On the other hand, when Ni is excessively contained, a ferrite phase becomes unstable, and the material cost increases. Therefore, the Ni content is 0.50 mass % or less. The Ni content is preferably 0.30 mass % or less.

(Carbon: C)

When C is excessively contained, the amount of a carbide in ferritic stainless steel increases, and the corrosion resistance of steel deteriorates. Therefore, the C content is 0.025 mass % or less. The C content is preferably 0.020 mass % or less.

The C content is preferably as small as possible and may be 0%; however, when the C content is decreased more than necessary, the cost increases, and thus the C content may be set to 0.002 mass % or more.

(Phosphorus: P)

When P is excessively contained, the workability of ferritic stainless steel deteriorates. Therefore, the P content is 0.040 mass % or less. The P content is preferably 0.030 mass % or less. The P content is preferably as small as possible and may be 0%; however, when the P content is decreased more than necessary, the cost increases, and thus the P content may be set to 0.001 mass % or more.

(Sulfur: S)

When S is excessively contained, the formation of a corrosion starting point is promoted in ferritic stainless steel. Therefore, the S content is 0.003 mass % or less. The S content is preferably 0.002 mass % or less. The S content is preferably as small as possible and may be 0%; however, when the S content is decreased more than necessary, the cost increases, and thus the S content may be set to 0.0001 mass % or more.

(Nitrogen: N)

When N is excessively contained, N forms a nitride with other element to cause ferritic stainless steel to be hard. Therefore, the N content is 0.025 mass % or less. The N content is preferably 0.020 mass % or less. The N content is preferably as small as possible and may be 0%; however, when the N content is decreased more than necessary, the cost increases, and thus the N content may be set to 0.003 mass % or more.

(Aluminum: Al)

Al is an effective element for improving the corrosion resistance of ferritic stainless steel and improving scale resistance. In addition, Al is an effective element as a deoxidizing agent at the time of steel making. In order to obtain these effects, the Al content is 0.002 mass % or more. The Al content is preferably 0.008 mass % or more.

On the other hand, when Al is excessively contained, there is a probability that the surface quality deteriorates. Therefore, the Al content is 0.200 mass % or less.

(Other Components)

The ferritic stainless steel according to one embodiment of the present invention may further contain one or mom of 0.01 mass % or more and 2.5 mass % or less of W, 0.01 mass % or more and 3.00 mass % or less of Mo, 0.001 mass % or more and 0.500 mass % or less of Ti, 0.0002 mass % or more and 0.0100 mass % or less of B, 0.0002 mass % or more and 0.0030 mass % or less of Ca, 0.001 mass % or more and 0.50 mass % or less of Hf, 0.01 mass % or more and 0.40 mass % or less of Zr, 0.005 mass % or more and 0.50 mass % or less of Sb, 0.01 mass % or more and 0.30 mass % or less of Co, 0.001 mass % or more and 1.0 mass % or less of Ta, 0.002 mass % or more and 1.00 mass % or less of Sn, 0.0002 mass % or more and 0.30 mass % or less of Ga, 0.01 mass % or more and 0.50 mass % or less of V, 0.001 mass % or more and 0.20 mass % or less of REM and 0.0003 mass % or more and 0.003 mass % or less of Mg. In addition, the ferritic stainless steel according to the present embodiment may contain, as REM, 0.20 mass % or less, preferably 0.10 mass % or less, of La or 0.20 mass % or less, preferably 0.05 mass % or less, of Ce.

Since it is not essential to contain these elements, the amounts thereof may be 0% and may be amounts below the ranges to be described below.

(Tungsten: W)

W is an element that may be contained to ensure high-temperature strength. In the case of obtaining this effect, the W content is preferably 0.01 mass % or more. The W content is more preferably 0.1 mass % or more.

On the other hand, when W is excessively contained, the material cost increases. Therefore, in the ferritic stainless steel according to the present embodiment, the W content is 2.5 mass % or less. The W content is preferably 1.5 mass % or less and more preferably 1.3 mass % or less.

(Molybdenum: Mo)

Mo is an element that may be contained to ensure high-temperature strength and red scale resistance. In the case of obtaining this effect, the Mo content is preferably 0.01 mass % or more.

On the other hand, when Mo is excessively contained, the ferritic stainless steel becomes hard, the workability deteriorates, and the material cost increases. Therefore, the Mo content is 3.00 mass % or less. The Mo content is preferably 2.50 mass % or less.

(Titanium: Ti)

Ti is an element that is capable of making ferritic stainless steel into a ferritic single phase at 900° C. to 1000° C. by reacting with C and/or N and improves red scale resistance and workability. Therefore, Ti may be contained. In the case of obtaining this effect, the Ti content is preferably 0.001 mass % or more. The Ti content is preferably 0.010 mass % or more and more preferably 0.050 mass % or more.

On the other hand, when Ti is excessively contained, there is a probability that workability and surface quality deteriorate. Therefore, the Ti content is 0.500 mass % or less. The T content is preferably 0.300 mass % or less and more preferably 0.250 mass % or less.

(Boron: B)

B is an element that improves the secondary workability of formed articles manufactured using ferritic stainless steel. In the case of obtaining this effect, the B content is preferably 0.0002 mass % or more.

On the other hand, when B is excessively contained, a compound such as Cr₂B is likely to be formed, and there is a probability that red scale resistance deteriorate. Therefore, the B content is 0.0100 mass % or less. The B content is preferably 0.0080 mass % or less or 0.0030 mass % or less.

(Calcium: Ca)

Ca is an element that promotes high-temperature oxidation resistance. Therefore, Ca may be contained as necessary. In the case of obtaining this effect, the Ca content is preferably 0.0002 mass % or more.

On the other hand, Ca being excessively contained causes the deterioration of corrosion resistance. Therefore, the Ca content is 0.0030 mass % or less.

(Hafnium: Hf)

Hf is an element that improves corrosion resistance, high-temperature strength and oxidation resistance. Hf may be contained as necessary. In the case of obtaining this effect, the Hf content is preferably 0.001 mass % or more. The Hf content is more preferably 0.01 mass % or more.

On the other hand, there is a concern that Hf being excessively contained may cause the deterioration of workability and manufacturability. Therefore, the Hf content is 0.50 mass % or less.

(Zirconium: Zr)

Zr is an element that improves high-temperature strength, corrosion resistance and high-temperature oxidation resistance. Therefore, Zr may be contained as necessary. In the case of obtaining this effect, the Zr content is preferably 0.01 mass % or more.

On the other hand, Zr being excessively contained causes the deterioration of workability and manufacturability. Therefore, the Zr content is 0.40 mass % or less.

(Antimony: Sb)

Sb is an element that improves high-temperature strength. Therefore, Sb may be contained as necessary. In the case of obtaining this effect, the Sb content is preferably 0.005 mass % or more. The Sb content is more preferably 0.01 mass % or more.

On the other hand, Sb being excessively contained degrades weldability and toughness. Therefore, the Sb content is 0.50 mass % or less.

(Cobalt: Co)

Co is an element that improves high-temperature strength. Therefore, Co may be contained as necessary. In the case of obtaining this effect, the Co content is preferably 0.01 mass % or more.

On the other hand, Co being excessively contained degrades toughness and thereby degrades manufacturability. Therefore, the Co content is 0.30 mass % or less.

(Tantalum: Ta)

Ta is an element that improves high-temperature strength. Therefore. Ta may be contained as necessary. In the case of obtaining this effect, the Ta content is preferably 0.001 mass % or more. The Ta content is more preferably 0.01 mass % or more and still more preferably 0.1 mass % or more.

On the other hand, Ta being excessively contained degrades weldability and toughness. Therefore, the Ta content is 1.0 mass % or less.

(Tin: Sn)

Sn is an element that improves corrosion resistance and high-temperature strength. Therefore, Sn may be contained as necessary. In the case of obtaining this effect, the Sn content is preferably 0.002 mass % or more. The Sn content is more preferably 0.01 mass % or more.

On the other hand, there is a concern that Sn being excessively contained may cause the deterioration of toughness and manufacturability. Therefore, the Sn content is 1.00 mass % or less.

(Gallium: Ga)

Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance. Therefore, Ga may be contained as necessary. In the case of obtaining this effect, the Ga content is preferably 0.0002 mass % or more. The Ga content is more preferably 0.01 mass % or more.

On the other hand, Ga being excessively contained degrades weldability and toughness. Therefore, the Ga content is 0.30 mass % or less.

(Vanadium: V)

V is an element that fixes solid solutions of C and N in steel as compounds and improves the ductility or workability of steel. Therefore, V may be contained as necessary. In the case of obtaining this effect, the V content is preferably 0.01 mass % or more.

On the other hand, V being excessively contained degrades the workability of steel. Therefore, the V content is 0.50 mass % or less.

(Magnesium: Mg)

Mg is a deoxidizing element and also an element that refines the structures of slabs to improve formability. Therefore, Mg may be contained as necessary. In the case of obtaining this effect, the Mg content is preferably 0.0003 mass % or more.

On the other hand, Mg being excessively contained causes deterioration of corrosion resistance, weldability and surface quality, and thus the Mg content is 0.003 mass % or less.

(Rare Earth Element: REM)

REM refers to a generic term for scandium (Sc) and 15 elements of lanthanum (La) through lutetium (Lu) (lanthanide elements). As REM, any one of the lanthanide elements may be contained singly or two or more lanthanide elements may be contained. In a case where any one lanthanide element is contained as REM, for example, as described below, any one of La and Ce may be contained or a lanthanide element other than La and Ce may be contained singly. In addition, in a case where two or more lanthanide elements are contained as REM, the combination of the elements is not particularly limited, and, as one example, La and Ce may be contained or a plurality of lanthanide elements that is contained in misch metal may be contained as REM by the addition of misch metal.

REM is an element that improves the cleanliness of stainless steel and improves high-temperature oxidation resistance. Therefore, REM may be contained as necessary. In the case of obtaining these effects, the REM content is preferably 0.001 mass % or more. The REM content is more preferably 0.01 mass % or more.

On the other hand, REM being excessively contained increases the alloying cost and degrades manufacturability. Therefore, the REM content is 0.20 mass % or less.

(Lanthanum: La)

La may be contained as REM. La is an element that improves the cleanliness of stainless steel and improves high-temperature oxidation resistance and, furthermore, an element that improves red scale resistance and scale exfoliation resistance. In a case where La is contained (using metallic La or the like) to obtain these effects, the La content is preferably set to 0.001 mass % or more. The La content is more preferably 0.01 mass % or more.

On the other hand, when La is excessively contained, the material cost increases. Therefore, the La content is 0.20 mass % or less. The La content is preferably 0.10 mass % or less, and, when the cost is taken into account, the La content is more preferably 0.05 mass % or less and still more preferably 0.03 mass % or less.

(Cerium: Ce)

Ce may be contained as REM. Ce is an element that improves the cleanliness of stainless steel and improves high-temperature oxidation resistance and, furthermore, an element that improves red scale resistance and scale exfoliation resistance. In a case where Ce is contained (using metallic Ce or the like) to obtain these effects, the Ce content is preferably 0.001 mass % or more. The Ce content is more preferably 0.01 mass % or more.

On the other hand, when Ce is excessively contained, the material cost increases. Therefore, the Ce content is 0.20 mass % or less. The Ce content is preferably 0.05 mass % or less.

The chemical composition of the ferritic stainless steel according to the present embodiment can be obtained from a depth position of ¼ of the sheet thickness from the surface (a position within a range of ⅛ to ⅜ of the thickness from the surface in the thickness direction is permitted) by performing an element analysis by an ordinary method such as ICP-AES. In addition, C and S may be measured using a combustion-infrared method, N may be measured using an inert gas melting-thermal conductivity method, and O may be measured using an inert gas melting-infrared absorption method.

<Oxides containing Al or Si>

In the ferritic stainless steel according to the present embodiment, at least one type of oxides of oxides containing 5 mass % or more of Al and oxides containing 5 mass % or more of Si are present on the surface, and, among the oxides present on the surface, the number of oxides having a diameter D of 0.1 μm or more and 2.0 μm or less (hereinafter, referred to as “Al/Si-based oxide”) is 10 or more per 93 μm², the diameter D being represented by the following formula (1).

These oxides improve red scale resistance.

D=(Dmax+Dmin)/2  (1)

(In the formula (1), Dmax is the maximum diameter of each oxide on the surface, and Dmin is the minimum diameter of each oxide on the surface.)

The dimensions of the oxides on the surface of the ferritic stainless steel can be measured with, for example, a scanning electron microscope (SEM).

Specifically, a SEM photograph of the surface of a steel material is captured using a scanning electron microscope (SEM). The area of one visual field is set to 93 μm². The maximum diameter and minimum diameter of each oxide are calculated from this SEM photograph with image analysis software, for example. “Photoshop (registered trademark)” (manufactured by Adobe Inc.).

Here, the amounts of Al and Si in each oxide on the surface of the ferritic stainless steel can be measured by, for example, energy dispersive X-ray spectroscopy (EDS). That is, EDS makes it possible to determine whether or not an oxide is the oxide to be counted for the number of the oxides (the oxides containing 5 mass % or more of Al or the oxides containing 5 mass % or more of Si).

In the present embodiment, the “maximum diameter” of an oxide means the maximum width between two parallel lines that sandwich the oxide when seen in a plan view. In addition, in the present specification, the “minimum diameter” of an oxide means the minimum width between two parallel lines that sandwich the oxide when seen in a plan view.

The reasons for the oxides containing 5 mass % or more of Al or the oxides containing 5 mass % or more of Si (referred to as the Al/Si-based oxides in some cases) to improve red scale resistance are considered as described below. First, the Al/Si-based oxide acts as a protective coating. Second, the Al/Si-based oxides grow by heating and the oxygen partial pressures around the Al/Si-based oxides decrease. Since Al, Si, Cr and Fe are more likely to be oxidized in this order, Al, Si and Cr are more preferentially oxidized than Fe. Therefore, the growth of the Al/Si-based oxides makes it possible to reduce the generation of red scale, which is a Fe-based oxide.

However, in a case where the Al/Si-based oxides is excessively present on the surface of the ferritic stainless steel, there is a concern that the Al/Si-based oxides may decrease the luminosity of the surface of the ferritic stainless steel and degrade the designability of the ferritic stainless steel. Therefore, the number of the Al/Si-based oxides on the surface of the ferritic stainless steel is preferably 25 or less and more preferably 22 or less per 93 μm². In this case, the luminosity on the surface of the ferritic stainless steel improves, and the designability can be kept favorable. In the present embodiment. “luminosity” means CIE1976 luminosity L* that is measured using a D65 light source in a diffuse lighting mode by receiving light in a direction at 8° with respect to the normal line to the surface of the ferritic stainless steel at a visual field angle of 10° visual field for a measurement time of one second.

On the surface of the ferritic stainless steel, in a case where the diameters D of the oxides containing 5 mass % or more of Al or Si are less than 0.1 μm, the effect of improving red scale resistance by the oxide is weak. Therefore, in the present embodiment, only oxides having a diameter D of 0.1 μm or more are taken into account.

On the other hand, when there are oxides having a diameter D of more than 2.0 μm among the oxides containing 5 mass % or more of Al or Si, there is a concern that the Al/Si-based oxides may decrease the luminosity of the surface of the ferritic stainless steel and degrade the designability of the ferritic stainless steel. Therefore, it is preferable to control the number of oxides that contain 5 mass % or more of Al or Si and have a diameter D of 0.1 μm or more and 2.0 μm or less on the surface of the ferritic stainless steel to a predetermined range.

In addition, on the surface of the ferritic stainless steel, the number of oxides that contain 5 mass % or more of Al or Si and have a diameter D of more than 2.0 μm is preferably small (for example, five or less per 93 μm²) and most preferably zero. In this case, the luminosity of the surface of the ferritic stainless steel is improved, and the designability of the ferritic stainless steel can be improved.

The present inventors paid attention to the Al/Si-based oxides on the surface of the ferritic stainless steel and came to obtain a knowledge that ferritic stainless steel having excellent red scale resistance can be realized by controlling the number of the Al/Si-based oxide to a predetermined range.

On the surface of the ferritic stainless steel according to the present embodiment, in addition to the Al/Si-based oxides, a passive film is present in a thickness of 2.0 to 8.0 nm. The passive film refers to a highly dense and highly adhesive coating composed of chromium oxyhydroxide hydrate mainly containing Cr and chromium oxide.

The thickness of the passive film can be obtained using a radio-frequency glow discharge spectroscopy (GDS). Specifically, the oxygen concentrations are analyzed at 2.5 nm pitches in the thickness direction from the surface using a GDS analyzer (for example, GD-Profiler 2 manufactured by Horiba, Ltd. or an equivalent device), a range from the surface to a position where the oxygen concentration shows a value that is half the peak value is defined as the passive film, and the thickness thereof is measured and obtained.

For example, other GDS measurement conditions are as described below.

-   -   Gas substitution time: 200 seconds,     -   Preliminary sputtering time: 30 seconds,     -   Background: 5 seconds,     -   Depth: 1.01 μm,     -   Pressure: 600 Pa,     -   Output: 35 W,     -   Effective value: 8.75 W,     -   Module: 8 V,     -   Phase: 4 V,     -   Frequency: 100 Hz duty cycle: 0.25.

In a case where the ferritic stainless steel according to the present embodiment is used at a high temperature where water vapor is contained, the Al/Si-based oxides grow, and the oxygen partial pressures around the Al/Si-based oxides are decreased. As a result, unlike normal ferritic stainless steel, the ferritic stainless steel has a surface form where the formation of an Fe-based oxide, which becomes an origin of red scale, has reduced and the formation of Cr, Al and Si-based oxides has increased and thereby has excellent red scale resistance.

For example, after the ferritic stainless steel is held in an atmosphere of 300° C. to 900° C. for 100 hours or longer, in a range from the surface to 1.0 μm (surface layer area), when the Cr content is represented by [Cr], the Si content is represented by [Si] and the Al content is represented by [Al] by unit mass %, the following formula (2) is satisfied.

[Cr]+[Si]+[Al]≥18.0  (2)

In a case where the formula (2) is not satisfied, it is considered that an Fe-based oxide has become dominant, that is, red scale has been excessively generated on the surface.

[Cr]+[Si]+[Al] is preferably 20.0 (mass %) or more.

There is no need to limit each of the Cr content [Cr], the Si content [Si] and the Al content [Al] in the range from the surface to 1.0 μm, but the Si content and/or the Al content are each preferably 3.0 mass % or more from the viewpoint of an effect of suppressing red scale.

In a high-temperature atmosphere, the Al/Si-based oxides grow slowly for a certain period of time; however, when 100 hours or longer has elapsed, the oxides do not change significantly, and thus it is considered that the presence states of Cr, Si and Al also do not change significantly.

That is, in a case where the ferritic stainless steel according to the present embodiment has been used as a component of an exhaust gas path material, stove combustion equipment, a fuel cell member, a plant-related material or the like, it is considered that the amounts of Cr, Al and Si in the surface layer area satisfy the formula (2).

Incidentally, even in a case where the ferritic stainless steel has been held as described above, the chemical composition at the ¼ depth position does not change.

<<Component for Exhaust Gas>>

A component for exhaust gas according to the present embodiment is obtained by working the ferritic stainless steel according to the present embodiment as a material. Therefore, in a stage of being obtained by working or the like (before being used as a component), the component for exhaust gas according to the present embodiment contains, as the chemical composition, 0.05 mass % or more and 2.50 mass % or less of Si, 0.05 mass % or more and 1.50 mass % or less of Mn, 0.025 mass % or less of C, 0.040 mass % or less of P, 0.003 mass % or less of S, 0.025 mass % or less of N, 0.01 mass % or more and 0.50 mass % or less of Ni, 10.50 mass % or more and 25.00 mass % or less of Cr, 0.01 mass % or more and 1.80 mass % or less of Cu, 0.002 mass % or more and 0.200 mass % or less of Al, 0.001 mass % or more and 1.00 mass % or less of Nb, 0 mass % or more and 2.5 mass % or less of W, 0 mass % or more and 3.00 mass % or less of Mo, 0 mass % or more and 0.500 mass % or less of Ti, 0 mass % or more and 0.0100 mass % or less of B, 0 mass % or more and 0.0030 mass % or less of Ca, 0 mass % or more and 0.50 mass % or less of Hf, 0 mass % or more and 0.40 mass % or less of Zr, 0 mass % or more and 0.50 mass % or less of Sb, 0 mass % or more and 0.30 mass % or less of Co, 0 mass % or more and 1.0 mass % or less of Ta, 0 mass % or more and 1.00 mass % or less of Sn, 0 mass % or more and 0.30 mass % or less of Ga, 0 mass % or more and 0.50 mass % or less of V, 0 mass % or more and 0.003 mass % or less of Mg and 0 mass % or more and 0.20 mass % or less of REM and contains Fe and an impurity as the remainder, at least one type of oxides of oxides containing 5 mass % or more of Al and oxides containing 5 mass % or more of Si are present on the surface, and, among the oxides present on the surface, the number of oxides having a diameter D of 0.1 μm or more and 2.0 μm or less is 10 or more per 93 μm², the diameter D being represented by the following formula (1).

D=(Dmax+Dmin)/2  (1)

In the formula (1). Dmax is the maximum diameter of the oxide on the surface, and Dmin is the minimum diameter of the oxide on the surface.

In addition, furthermore, there are also cases where, regarding the surface, CIE1976 luminosity L* that is measured using a D65 light source in a diffuse lighting mode by receiving light in a direction at 8° with respect to the normal line to the surface at a visual field angle of 10° visual field for a measurement time of one second is 60 or more.

In addition, after this component for exhaust gas according to the present embodiment is held in an atmosphere of 300° C. to 900° C. for 100 hours or longer, in a range from the surface to 1.0 μm, when the Cr content is represented by [Cr], the Si content is represented by [Si] and the Al content is represented by [Al] by unit mass %, the following formula (2) is satisfied.

[Cr]+[Si]+[Al]≥18.0  (2)

That is, the component for exhaust gas according to the present embodiment is, for example, a component of exhaust gas path materials, stove combustion equipment, fuel cell members, plant-related materials or the like, and, in a case where the component for exhaust gas has been used for a certain period of time under normal conditions in such applications (after use), in a range from the surface to 1.0 μm, when the Cr content is represented by [Cr], the Si content is represented by [Si] and the Al content is represented by [Al] by unit mass %, the following formula (2) is satisfied.

[Cr]+[Si]+[Al]≥18.0  (2)

In any of the ferritic stainless steel according to the present embodiment and the component for exhaust gas according to the present embodiment, the Cr content [Cr], the Si content [Si] and the Al content [Al] in the range from the surface to 1.0 μm can be measured using GDS.

Specifically, the analysis region is set to 4 mm, and all elements other than C and N that are contained in each steel are selected and measured at 2.5 nm pitches up to a depth of 1.0 μm. From the measurement results, the amount of each of Cr, Al and Si at a position where each of Cr, Al and Si shows a peak in a depth range of up to 1.0 μm is calculated.

<<Manufacturing Method of Ferritic Stainless Steel>>

Conventionally, as a method for improving red scale resistance, a method in which the diffusion of Cr in steel is promoted by surface polishing as finishing and the formation of an oxide of Cr is accelerated, a method in which a hot-dip plating layer is formed on a surface layer or the like has been used.

The present inventors found that ferritic stainless steel having excellent red scale resistance in which the number of the Al/Si-based oxides on the surface is 10 or more per 93 μm² can be obtained by, for example, the following manufacturing method.

The ferritic stainless steel in one embodiment of the present invention is obtained as, for example, a ferritic stainless-steel strip. FIG. 1 is a flowchart showing an example of the manufacturing method of ferritic stainless steel according to the present embodiment. As shown in FIG. 1 , a manufacturing method of a ferritic stainless-steel strip in the present embodiment includes a pretreatment step S1, a hot rolling step S2, an annealing step S3, a first pickling step S4, a cold rolling step S5, a final annealing step S6, a nitric acid electrolysis step S7 and a final pickling step S8.

Preferable conditions for each step will be described. Hereinafter, regarding conditions that will not be described, well-known conditions can be adopted.

<Pretreatment Step>

In the pretreatment step S1, first, steel having a chemical composition adjusted so as to be in the above-described range of the present invention is melted using a melting furnace with a vacuum or argon atmosphere, and this steel is cast to manufacture a slab. After that, a slab piece for hot rolling is cut out from the slab. Then, the slab piece is heated to a temperature range of 1100° C. to 1300° C. in the atmosphere. The time during which the slab piece is heated and held is not limited. In the case of performing the pretreatment step industrially, the casting may be continuous casting.

<Hot Rolling Step>

The hot rolling step S2 is a step of manufacturing a hot-rolled steel strip having a predetermined thickness by the hot rolling of the slab (steel ingot) that is obtained in the pretreatment step S1. The conditions for the hot rolling are not limited and may be adjusted depending on required mechanical characteristics or the like.

<Annealing Step>

The annealing step S3 is a step of softening the steel strip by heating the hot-rolled steel strip obtained in the hot rolling step S2. This annealing step S3 is a step that is performed as necessary and may not be performed.

<First Pickling Step>

The first pickling step S4 is a step of washing off scale adhering to the surface of the steel strip using a pickling liquid such as a liquid mixture of hydrochloric acid or nitric acid and hydrofluoric acid.

<Cold Rolling Step>

The cold rolling step S5 is a step of rolling the steel strip from which scale has been removed in the first pickling step S4 to be thinner.

<Final Annealing Step>

The final annealing step S6 is a step of heating the steel strip rolled to be thin in the cold rolling step S5 to remove strain and softening the steel strip. In addition, the final annealing step is a step of forming internal oxides, which are oxides of Al, Si or the like, together with outer layer oxides such as (Fe, Cr)₃O₄ or Cr₂O₃.

For the above-described purpose, annealing in the final annealing step S6 is performed at a temperature of approximately 900° C. to 1100° C. for a time within a range of 30 to 90 seconds in, as an atmosphere, the atmosphere or a combustion gas atmosphere of liquefied combustion gas (LNG) or the like depending on alloy components.

<Nitric Acid Electrolysis Step>

The nitric acid electrolysis step S7 is a step of performing an electrolytic treatment on the steel strip obtained in the final annealing step S6 in a nitric acid aqueous solution. In the nitric acid electrolysis step S7, oxides adhering to the surface of the steel strip are partially removed.

Specifically, on the surface of the steel strip obtained in the final annealing step S6, for example, the outer layer oxides such as (Fe. Cr)₃O₄ or Cr₂O₃ have been formed. In addition, the internal oxides, which are mainly oxides of Al, Si or the like, have been formed between these outer layer oxides and the base material. In the nitric acid electrolysis step S7, nitric acid electrolysis is performed under conditions where a majority of the internal oxides remain while a majority of the outer layer oxides are removed. While the majority of the internal oxides remain, it is preferable that some of the internal oxides slightly exfoliate and are put into a state of being easily removable by the final pickling step S8.

The nitric acid concentration in the nitric acid electrolysis step S7 is preferably 150 g/L or lower. In this case, it is easy to leave 10 or more Al/Si-based oxides per 93 μm² on the surface of the steel strip after the final pickling step S8.

On the other hand, when more Al/Si-based oxides than necessary remain, the luminosity of the surface decreases. Therefore, the nitric acid concentration in the nitric acid electrolysis step S7 is preferably 100 g/L or higher in order to obtain an amount of the Al/Si-based oxides on the surface after the final pickling step in a preferable range. In addition, the nitric acid concentration in the nitric acid electrolysis step S7 is preferably 130 g/L or higher in order to efficiently remove the outer layer oxides within a short period of time.

The liquid temperature in the nitric acid electrolysis step S7 is preferably 70° C. or lower and more preferably 60° C. or lower. In this case, it is easy to leave 10 or more Al/Si-based oxides per 93 μm² on the surface of the steel strip after the final pickling step S8.

On the other hand, the liquid temperature in the nitric acid electrolysis step S7 is preferably 50° C. or higher and more preferably 60° C. or higher. In this case, it is possible to efficiently remove the outer layer oxides within a short period of time.

The current density in the nitric acid electrolysis step S7 is preferably 150 mA/cm² or lower. In this case, it is easy to leave 10 or more Al/Si-based oxides per 93 μm² on the surface of the steel strip after the final pickling step S8.

On the other hand, the current density in the nitric acid electrolysis step S7 is preferably 100 mA/cm² or higher, more preferably 120 mA/cm² or higher and still more preferably 130 mA/cm² or higher. In this case, it is possible to efficiently remove the outer layer oxides within a short period of time.

The electrolysis time in the nitric acid electrolysis step S7 is more preferably 120 seconds or shorter. In this case, it is possible to leave 10 or more Al/Si-based oxides per 93 μm² on the surface of the steel strip after the final pickling step S8.

On the other hand, the electrolysis time in the nitric acid electrolysis step S7 is preferably 60 seconds or longer. In this case, it is possible to reliably remove a majority of the outer layer oxides, put some of the internal oxides into a state of being easily exfoliated and obtain an amount of the Al/Si-based oxides remaining after the final pickling step in a preferable range.

<Final Pickling Step>

The final pickling step S8 is a step of immersing the steel strip after the nitric acid electrolysis step S7 into a pickling liquid such as a liquid mixture of nitric acid and hydrofluoric acid. In the final pickling step S8, the internal oxides that have slightly exfoliated in the nitric acid electrolysis step S7 are removed. This makes it possible to remove Al/Si-based oxides that are present more than necessary while ensuring 10 or more Al/Si-based oxides per 93 μm² on the surface of the steel strip and to improve the luminosity.

As described above, in the related art, a step such as polishing finish or the formation of a plating layer is further performed as a finishing step for improving red scale resistance. However, such a finishing step has a problem in that there is a need to introduce a new device for the finishing step and the manufacturing cost becomes high. From such a viewpoint, there is a demand for a manufacturing method for manufacturing ferritic stainless steel having excellent red scale resistance without increasing the manufacturing cost.

In the above-described manufacturing method according to the present embodiment, since a device that is ordinarily used in pickling steps can be used in the nitric acid electrolysis step S7 and the final pickling step S8, it is possible to realize ferritic stainless steel having excellent red scale resistance without increasing the manufacturing cost.

<Manufacturing Method of Component for Exhaust Gas>

The component for exhaust gas according to the present embodiment can be obtained by working the above-described ferritic stainless steel according to the present embodiment into a predetermined component shape by a well-known working method.

Examples

Examples of the present invention will be described below. First, the steps of the above-described manufacturing method up to the final annealing step S6 were performed all under the same conditions using components shown in Table 1 below as raw materials, the nitric acid electrolysis step S7 was performed for electrolysis times shown in Table 2, and then the final pickling step S8 was performed, thereby manufacturing ferritic stainless steel.

In the present examples, the composition of each stainless steel shown in Table 1 is indicated by “mass %”. In addition, a remainder other than each component shown in Table 1 is Fe and an impurity. In addition, underlines in Table 1 indicate that the range of each component that is contained in stainless steel according to each comparative example of the present invention is outside the range of the present invention.

TABLE 1 Kind of steel C Si Mn P S Ni Cr N Nb Mo Cu Al Ti Others A1 0.004 0.14 0.09 0.027 0.001 0.11 17.45 0.009 0.004 1.01 0.03 0.120 0.191 Ca: 0.0004, Ga: 0.09 A2 0.008 1.12 1.06 0.027 0.001 0.12 13.88 0.010 0.39 — 0.13 0.033 0.002 Hf: 0.15 A3 0.009 0.23 0.98 0.027 0.002 0.15 18.34 0.008 0.65 2.05 0.19 0.019 0.010 Zr: 0.02, Sb: 0.03 A4 0.005 0.25 0.25 0.029 0.001 0.35 17.12 0.008 0.53 0.06 1.43 0.094 0.150 B: 0.0008 A5 0.007 0.56 0.20 0.028 0.001 0.12 18.54 0.018 0.44 0.05 0.45 0.023 0.057 Co: 0.01, Ta: 0.1 A6 0.012 0.43 0.33 0.031 0.001 0.17 22.09 0.011 0.20 1.04 0.23 0.069 0.200 Sn: 0.05, La: 0.03, Ce: 0.01 A7 0.008 0.38 0.88 0.025 0.001 0.08 17.12 0.009 0.47 1.99 1.52 0.021 — W: 1, 3 A8 0.008 0.21 0.08 0.021 0.001 0.23 17.33 0.008 0.002 0.98 0.02 0.190 0.210 REM: 0.06, Mg: 0.003 A9 0.007 0.19 0.10 0.021 0.001 0.18 17.53 0.008 0.001 0.92 0.02 0.090 0.190 — A10 0.006 0.35 0.34 0.026 0.001 0.19 17.22 0.009 0.410 0.01 0.05 0.150 0.003 V: 0.35 B1 0.009 0.04 0.88 0.031 0.001 0.11 14.02 0.008 0.41 — 0.13 0.023 — — B2 0.007 0.42 0.33 0.028 0.002 0.23  9.81 0.009 0.43 — 0.11 0.021 0.150 — B3 0.007 0.69 0.29 0.028 0.001 0.11 11.99 0.010 0.01 — 0.01 0.001 0.240 —

As shown in Table 1, ferritic stainless steels produced so as to have a chemical composition within the range of the present invention were defined as kinds of steel A1 to A10. In addition, ferritic stainless steels produced so as to have a chemical composition outside the range of the present invention were defined as kinds of steel B1 to B3.

Table 2 is a table showing conditions that were used to manufacture steel materials of steel materials Nos. 1 to 46 using the kinds of steel A1 to A10 and the kinds of steel B1 to B3 and the evaluation results of each steel material. Conditions used upon the manufacturing of each steel material shown in Table 2 are as described below.

-   -   Atmosphere of melting furnace in pretreatment step S1 Vacuum     -   Mass of slab piece that was manufactured in pretreatment step S1         30 kg     -   Heating temperature of slab piece in pretreatment step S1 1230°         C.     -   Heating time of slab piece in pretreatment step S1 2 hours     -   Sheet thickness after hot rolling step S2 4 mm     -   Annealing step S3 Not performed     -   Pickling liquid used in first pickling step S4 Nitric         hydrofluoric acid (aqueous solution with a hydrofluoric acid         concentration of 30 g/L and a nitric acid concentration of 100         g/L)     -   Liquid temperature in first pickling step S4 40° C. to 50° C.     -   Sheet thickness after cold rolling step S5 1.5 mm     -   Annealing temperature in final annealing step S6 900° C. to         1100° C. (changed depending on alloy composition)     -   Annealing time in final annealing step S6 60 seconds     -   Annealing atmosphere in final annealing step S6 Atmosphere     -   Nitric acid concentration in nitric acid electrolysis step S7         150 g/L     -   Liquid temperature in nitric acid electrolysis step S7 50° C. to         70° C.     -   Current density in nitric acid electrolysis step S7 150 mA/cm²     -   Electrolysis time in nitric acid electrolysis step S7 30 to 180         seconds (shown in Table 2)     -   Pickling liquid used in final pickling step S8 Nitric         hydrofluoric acid (aqueous solution with a hydrofluoric acid         concentration of 20 g/L and a nitric acid concentration of 70 to         80 g/L)     -   Liquid temperature in final pickling step S8 40° C. to 50° C.

TABLE 2 Number of Al/Si- Electrol- based Steel Kind ysis oxides Weight material of time (oxides/ gain Lumi- Formula No. steel (seconds) 93 μm²) (mg/cm²) nosity (2) 1 A1 30 48 0.01 39 35.2 2 60 21 0.01 70 28.9 3 120 19 0.01 77 27.5 4 180  1 4.23 86 8.7 5 A2 40 36 0.01 58 — 6 60 19 0.01 75 — 7 120 15 0.01 83 — 8 180  2 4.67 91 — 9 A3 40 28 0.01 46 — 10 60 16 0.01 69 — 11 120 12 0.01 72 — 12 180  1 3.77 81 — 13 A4 40 41 0.01 52 36.7 14 60 25 0.01 71 33.4 15 120 16 0.01 79 26.7 16 180  2 4.86 83 7.1 17 A5 40 29 0.01 55 — 18 60 19 0.01 68 — 19 120 14 0.01 73 — 20 180  3 4.23 85 — 21 A6 40 32 0.01 35 — 22 60 21 0.01 61 — 23 120 16 0.01 73 — 24 180  4 0.01 79 — 25 A7 40 29 0.01 43 — 26 60 18 0.01 64 — 27 120 13 0.01 73 — 28 180  2 4.43 80 — 29 A8 60 22 0.01 61 — 30 120 18 0.01 67 — 31 180  5 0.96 73 — 32 A9 40 27 0.01 41 29.2 33 60 20 0.01 68 27.8 34 180  2 3.55 80 8.8 35 A10 60 25 0.01 68 — 36 120 19 0.01 70 — 37 B1 60  9 4.09 78 11.8 38 120  6 4.67 83 8.1 39 180  0 4.12 90 9.5 40 B2 60  8 5.23 80 — 41 120  2 5.32 89 — 42 180  0 5.08 91 — 43 B3 40 24 4.98 50 — 44 60 20 4.88 55 — 45 120  5 4.77 73 — 46 180  1 4.67 86 —

Regarding the steel materials No. 1 to 46, the numbers of Al/Si-based oxides were measured by a method to be described below in detail. The measurement results are shown in Table 2.

<Number of Al/Si-Based Oxides>

The number of Al/Si-based oxides on the surface of the steel material was measured as described below. First, a SEM photograph of the surface of the steel material was captured using a scanning electron microscope (SEM) SU5000 (manufactured by Hitachi High-Tech Corporation) at a magnification of 10000 fold. The dimensions of one visual field was 8.34 μm in length and 11.2 μm in width, and the area of one visual field was 93 μm². In addition, regarding the compositions of the oxides, an element analysis was performed using energy dispersive X-ray spectroscopy (EDS) (manufactured by Horiba, Ltd.) at an accelerating voltage of 15 kV for an analysis time of 60 seconds.

An image analysis was performed on the captured SEM photograph with image analysis software “Photoshop (registered trademark)” (manufactured by Adobe Inc.), and the number of oxides that contained 5 mass % or more of Al or Si and had a diameter D of 0.1 μm or more and 2.0 μm or less, that is, the number of Al/Si-based oxides, was calculated.

In addition, the thicknesses of passive films were measured by the above-described method. While not shown in the tables, the thicknesses of passive films were 2.0 to 8.0 nm.

FIG. 2A and FIG. 2B show examples of the SEM photographs captured by the above-described method. As shown in FIG. 2A, the number of Al/Si-based oxides present on the surface of the steel material No. 6, which is an invention example, was 10 or more in one visual field. In contrast, as shown in FIG. 2B, the number of Al/Si-based oxides present on the surface of the steel material No. 8, which is a comparative example, was less than 10 in one visual field. In the SEM photograph of the steel material No. 6, 30 or more oxides seem to be present in one visual field. However, the number of oxides that contained 5 mass % or more of Al or Si and had a diameter D of 0.1 μm or more and 2.0 μm or less, that is, the number of the Al/Si-based oxides, was 19 in one visual field.

As shown in Table 2, in the steel materials Nos. 1 to 3, 5 to 7, 9 to 11, 13 to 15, 17 to 19, 21 to 23, 25 to 27, 29, 30, 32, 33, 35 and 36 in which the kinds of steel A1 to A10 were used and the electrolysis times were 30 seconds to 120 seconds, the numbers of Al/Si-based oxides having a diameter D of 0.1 μm or more and 2.0 μm or less were 10 or more per 93 μm².

On the other hand, in the steel materials Nos. 4, 8, 12, 16, 20, 24, 28, 31 and 34 in which the kinds of steel A1 to A9 were used but the electrolysis times were 180 seconds, the numbers of Al/Si-based oxides were less than 10 per 93 μm².

On the other hand, in the steel materials Nos. 37 to 42 in which the kind of steel B1 or B2 was used, the numbers of Al/Si-based oxides were less than 10 per 93 μm².

In the steel materials Nos. 43 and 44 in which the kind of steel B3 was used and the electrolysis times were 40 to 60 seconds, the numbers of Al/Si-based oxides were 10 or more per 93 μm²; however, in the steel materials Nos. 45 and 46 in which the kind of steel B3 was used and the electrolysis times were 120 to 180 seconds, the number of Al/Si-based oxides were less than 10 per 93 μm².

<Weight Gain (Red Scale Resistance Evaluation)>

In order to evaluate red scale resistance regarding the steel materials Nos. 1 to 46, the weight gain of the steel materials was measured as described below according to JIS Z 2281: 1993 (Test method for continuous oxidation test at elevated temperatures for metallic materials).

First, a 20 mm×25 mm test piece was cut out from each steel material. This test piece the test piece was continuously heated at 600° C. for 100 hours in an atmospheric environment where the water vapor concentration was 10 vol % with an assumption of a status where a petroleum-based fuel had been combusted. The weight gain was calculated from a change in mass before and after the test.

As a criterion for red scale resistance evaluation, when the weight gain was 0.20 mg/cm² or less, the red scale resistance was determined to be excellent.

As shown in Table 2, in the steel materials Nos. 1 to 3, 5 to 7, 9 to 11, 13 to 15, 17 to 19, 21 to 23, 25 to 27, 29, 30, 32, 33, 35 and 36 in which the kinds of steel A1 to A10 were used and the numbers of the Al/Si-based oxides were 10 or more per 93 μm², the weight gain was 0.20 mg/cm² or less. Therefore, it was indicated that red scale resistance was favorable in these steel materials.

On the other hand, in the steel materials Nos. 4, 8, 12, 16, 20, 24, 28, 31 and 34 in which the kinds of steel A1 to A9 were used and the numbers of the Al/Si-based oxides were less than 10 per 93 μm², the weight gain exceeded 0.20 mg/cm². Therefore, it was indicated that red scale resistance was low in these steel materials.

In addition, in the steel materials Nos. 37 to 42 in which the kind of steel B1 or B2 was used and the numbers of the Al/Si-based oxides were less than 10 per 93 μm², the weight gain exceeded 0.20 mg/cm². Therefore, it was indicated that red scale resistance was low in these steel materials.

In all of the steel materials Nos. 43 to 46 in which the kind of steel B3 was used, the weight gain exceeded 0.20 mg/cm² regardless of the number of the Al/Si-based oxides. Therefore, it was indicated that red scale resistance was low in these steel materials. In the kind of steel B3, since the Al content was as low as 0.001 mass %, it is assumed that the red scale resistance was low even when the number of the Al/Si-based oxides was 10 or more.

<Surface Form after Long Duration Test>

In order to evaluate the form of the oxides on the surface after the long duration test, regarding the steel materials Nos. 1 to 4, 13 to 16, 32 to 34 and 37 to 39 among the test pieces for which the weight gain (red scale resistance evaluation) had been evaluated, the Cr, Si and Al contents at an arbitrary place in the test piece were measured using glow discharge spectroscopy (GDS) (GD-Profiler 2 manufactured by Horiba, Ltd.). An analysis region was set to +4 mm, and the Cr, Si and Al contents were measured at 2.5 nm pitches up to a depth of 1.0 μm from the surface.

As analysis elements, all elements other than C and N that were contained in each steel at the arbitrary place were selected and measured.

Other GDS measurement conditions were as described below.

-   -   Gas substitution time: 200 seconds,     -   Preliminary sputtering time: 30 seconds,     -   Background: 5 seconds,     -   Depth: 1.01 μm.     -   Pressure: 600 Pa,     -   Output: 35 W,     -   Effective value: 8.75 W,     -   Module: 8 V,     -   Phase: 4 V,     -   Frequency: 100 Hz duty cycle: 0.25.

After the measurement, the amount of each of Cr, Al and Si at a position where each element showed a peak within a depth of 1.0 μm was assigned to the formula (2) to calculate a value, and whether or not the value satisfied the formula (2) was confirmed. In the table, the values of the left side ([Cr]+[Si]+[Al]) of the formula (2) are shown.

As shown in Table 2, in Nos. 1 to 3, 13 to 15, 32 and 33 in which the weight gain was 0.20 mg/cm² or less and the red scale resistance was good, the formula (2) was satisfied.

On the other hand, in Nos. 4, 16, 34 and 37 to 39 in which the weight gain was more than 0.20 mg/cm² and the red scale resistance was poor, the formula (2) was not satisfied.

<Luminosity>

In order to evaluate the designability of the steel materials, the luminosity on the surfaces of the steel materials Nos. 1 to 46 was measured in the following way. After white calibration was performed at room temperature of 23° C. using a spectrophotometric colorimeter (Model No.: CM-700d, manufactured by Konica Minolta, Inc.), the luminosity L* on the surface of the steel material was measured at the same temperature. As an evaluation criterion, when the luminosity L* was 60 or more, the designability was determined to be excellent.

As shown in Table 2, in the steel materials Nos. 2 to 4, 6 to 8, 10 to 12, 14 to 16, 18 to 20, 22 to 24, 26 to 28, 29 to 31, 33 and 34 in which the kinds of steel A1 to A10 were used and the electrolysis times were 60 seconds to 180 seconds, the luminosity L* was 60 or more. Therefore, it was indicated that designability was favorable in these steel materials.

On the other hand, in the steel materials Nos. 1, 5, 9, 13, 17, 21, 25 and 32 in which the kinds of steel A1 to A9 were used and the electrolysis times were 30 to 40 seconds, the luminosity L* was less than 60.

ADDITIONAL REMARK

The present invention is not limited to each of the above-described embodiments and can be modified in a variety of manners within the scope of the claims, and embodiments that can be obtained by appropriately combining technical means individually disclosed in different embodiments are also included in the technical scope of the present invention. 

1. A ferritic stainless steel comprising, as a chemical composition: 0.05 mass % or more and 2.50 mass % or less of Si; 0.05 mass % or more and 1.50 mass % or less of Mn; 0.025 mass % or less of C; 0.040 mass % or less of P; 0.003 mass % or less of S; 0.025 mass % or less of N; 0.01 mass % or more and 0.50 mass % or less of Ni; 10.50 mass % or more and 25.00 mass % or less of Cr; 0.01 mass % or more and 1.80 mass % or less of Cu; 0.002 mass % or more and 0.200 mass % or less of Al; 0.001 mass % or more and 1.00 mass % or less of Nb; 0 mass % or more and 2.5 mass % or less of W; 0 mass % or more and 3.00 mass % or less of Mo; 0 mass % or more and 0.500 mass % or less of Ti; 0 mass % or more and 0.0100 mass % or less of B; 0 mass % or more and 0.0030 mass % or less of Ca; 0 mass % or more and 0.50 mass % or less of Hf; 0 mass % or more and 0.40 mass % or less of Zr; 0 mass % or more and 0.50 mass % or less of Sb; 0 mass % or more and 0.30 mass % or less of Co; 0 mass % or more and 1.0 mass % or less of Ta; 0 mass % or more and 1.00 mass % or less of Sn; 0 mass % or more and 0.30 mass % or less of Ga; 0 mass % or more and 0.50 mass % or less of V; 0 mass % or more and 0.003 mass % or less of Mg; 0 mass % or more and 0.20 mass % or less of REM; and a remainder including Fe and an impurity, wherein at least one type of oxides of oxides containing 5 mass % or more of Al and oxides containing 5 mass % or more of Si are present on a surface, and, among the oxides present on the surface, the number of oxides having a diameter D of 0.1 μm or more and 2.0 μm or less is 10 or more per 93 μm², the diameter D being represented by the following formula (1) D=(Dmax+Dmin)/2  (1) in the formula (1), Dmax is a maximum diameter of the oxide on the surface, and Dmin is a minimum diameter of the oxide on the surface.
 2. The ferritic stainless steel according to claim 1, wherein, regarding the surface, CIE1976 luminosity L* that is measured using a D65 light source in a diffuse lighting mode by receiving light in a direction at 8° with respect to a normal line to the surface at a visual field angle of 10° visual field for a measurement time of one second is 60 or more.
 3. The ferritic stainless steel according to claim 1, wherein the chemical composition contains one or more of: 0.01 mass % or more and 2.5 mass % or less of W; 0.01 mass % or more and 3.00 mass % or less of Mo; 0.001 mass % or more and 0.500 mass % or less of Ti; 0.0002 mass % or more and 0.0100 mass % or less of B; 0.0002 mass % or more and 0.0030 mass % or less of Ca; 0.001 mass % or more and 0.50 mass % or less of Hf; 0.01 mass % or more and 0.40 mass % or less of Zr; 0.005 mass % or more and 0.50 mass % or less of Sb; 0.01 mass % or more and 0.30 mass % or less of Co; 0.001 mass % or more and 1.0 mass % or less of Ta; 0.002 mass % or more and 1.00 mass % or less of Sn; 0.0002 mass % or more and 0.30 mass % or less of Ga; 0.01 mass % or more and 0.50 mass % or less of V; 0.0003 mass % or more and 0.003 mass % or less of Mg; and 0.001 mass % or more and 0.20 mass % or less of REM.
 4. The ferritic stainless steel according to claim 1, wherein, in a range from the surface to 1.0 μm after the ferritic stainless steel is held in an atmosphere of 300° C. to 900° C. for 100 hours or longer, when a Cr content is represented by [Cr], a Si content is represented by [Si] and an Al content is represented by [Al] by unit mass %, the following formula (2) is satisfied, [Cr]+[Si]+[Al]≥18.0  (2).
 5. A ferritic stainless steel, wherein, in a range from a surface to 1.0 μm, when a Cr content is represented by [Cr], a Si content is represented by [Si] and an Al content is represented by [Al] by unit mass %, the following formula (2) is satisfied, [Cr]+[Si]+[Al]≥18.0  (2).
 6. A component for exhaust gas comprising: the ferritic stainless steel according to claim
 1. 7. The ferritic stainless steel according to claim 2, wherein, in a range from the surface to 1.0 μm after the ferritic stainless steel is held in an atmosphere of 300° C. to 900° C. for 100 hours or longer, when a Cr content is represented by [Cr], a Si content is represented by [Si] and an Al content is represented by [Al] by unit mass %, the following formula (2) is satisfied, [Cr]+[Si]+[Al]≥18.0  (2).
 8. The ferritic stainless steel according to claim 3, wherein, in a range from the surface to 1.0 μm after the ferritic stainless steel is held in an atmosphere of 300° C. to 900° C. for 100 hours or longer, when a Cr content is represented by [Cr], a Si content is represented by [Si] and an Al content is represented by [Al] by unit mass %, the following formula (2) is satisfied, [Cr]+[Si]+[Al]≥18.0  (2).
 9. A component for exhaust gas comprising: the ferritic stainless steel according to claim
 2. 10. A component for exhaust gas comprising: the ferritic stainless steel according to claim
 3. 11. A component for exhaust gas comprising: the ferritic stainless steel according to claim
 4. 12. A component for exhaust gas comprising: the ferritic stainless steel according to claim
 5. 13. A component for exhaust gas comprising: the ferritic stainless steel according to claim
 7. 14. A component for exhaust gas comprising: the ferritic stainless steel according to claim
 8. 